1. Field of the Invention
The present invention relates to a negative active material for a rechargeable lithium battery, a method of preparing the same, and a rechargeable lithium battery including the same. More particularly, the present invention relates to a negative active material capable of improving the cycle-life characteristics and initial efficiency of high-capacity rechargeable lithium batteries.
2. Description of the Related Art
The use of portable electronic instruments is increasing as electronic equipment gets smaller and lighter due to developments in high-tech electronic industries. Therefore, studies of high-capacity negative active materials are actively being pursued in accordance with an increased need for batteries having high energy density for use as power sources in these portable electronic instruments. Even though graphite is suggested for the negative active material as it has a theoretical capacity of 372 mAh/g, a novel material having a higher capacity than that of graphite is still needed.
Elemental materials such as Si, Sn, and Al have been developed as substitutes for graphite. These elemental materials are known to alloy with lithium and have higher electric capacities than graphite.
However, elements themselves have not yet been commercialized as negative active materials because the elements (such as Si, Sn, Al, and so on) form alloys with lithium during charge-discharge and undergo volume expansion and shrinkage resulting in element pulverization. As a result, the cycle-life of the battery may decrease.
Recently, certain materials have been proposed as substitutes for the conventional graphite material. One such substitute includes a simple mixture of a graphite and silicon compound powder. Another proposed substitute includes a material in which a pulverized silicon compound is chemically fixed on the surface of graphite by a silane coupling agent. A third substitute includes a material in which a metal such as Si is bound with or coated on a graphite-based carbonaceous material.
However, in the simple mixture of a graphite and silicon compound powder, the graphite does not completely contact the silicon compound. As a result, the silicon compound is released from the graphite when the graphite is expanded or contracted upon repeated charge and discharge cycles. Thereby, as the silicon compound has low electro-conductivity, the silicon compound is insufficiently utilized for negative active materials and the cycle characteristics of the rechargeable lithium battery are deteriorated.
In addition, the material in which the pulverized silicon compound is chemically fixed on the surface of graphite by a silane coupling agent works as a negative active material (similar to graphite) at the early charge and discharge cycles. However, the silicon compound expands when it is alloyed with the lithium upon repeated charge and discharge cycles. Thereby, the linkage of the silane coupling agent is broken to release the silicon compound from the graphite such that the silicon compound is insufficiently utilized as a negative active material. As a result, the cycle characteristics of the rechargeable lithium battery are deteriorated. Further, the silane coupling agent may not be uniformly treated upon preparing the negative electrode material so that it is difficult to provide a negative electrode material having consistent quality.
Further, the material in which a metal such as Si is bound with or coated on the graphite-based carbonaceous material has the same problems. That is, upon repeated charge and discharge cycles, the linkage of the amorphous carbonaceous material is broken upon expanding the metal alloyed with the lithium. Thereby, the metal is separated and thus is not sufficiently utilized as a negative active material. As a result, cycle-life characteristics of the lithium rechargeable battery are deteriorated.